1. Field of the Invention
The present invention relates to a wave absorber. More particularly, the present invention relates to a wave absorber which is lightweight and superior in self-extinguishing property, and shows an excellent wave-absorbing property owing to a uniformly dispersed dielectric loss material.
2. Prior Art
Anechoic chambers have been widely used for applications such as measurement of properties of antennas, testing of electromagnetic field intensity testers, measurement of jamming wave intensity, and the like. Each anechoic chamber is provided, at the wall, ceiling, floor, etc., with a wave absorber(s) (an example thereof is shown in FIG. 1) having, at the surface, a wave-absorbing portion consisting of a plurality of projections of circular cone shape, triangular pyramid shape, square pyramid shape, wedge shape or the like.
As such a wave absorber, there is in general and wide use one comprising (1) a foamed resin having the above-mentioned projections, consisting of a foamed polystyrene resin, a foamed polyurethane resin, a foamed polyethylene resin or the like and (2) a dielectric loss material (e.g. carbon black) dispersed in the foamed resin (1). The wave absorber is used in an anechoic chamber by adhering its back side to the wall, floor and ceiling of the anechoic chamber with an adhesive or the like.
Of various wave absorbers each comprising a different foamed resin, a wave absorber comprising a foamed polystyrene resin has the following problems. This wave absorber is obtained by adding a dielectric loss material (e.g. carbon black) and a binder to prefoamed polystyrene resin beads and then molding the mixture. Since the beads are particles of relatively large diameters (about 0.1-1 mm), the resulting foamed material has large-diameter cells, which requires addition of a large amount of a dielectric loss material (e.g. carbon black) in order for the final wave absorber to exhibit sufficiently high wave absorbability. Moreover, the wave absorber using a foamed polystyrene resin is applicable only in a relatively low-frequency zone and cannot be used in a high-frequency zone of, for example, 10 GHz or higher.
Meanwhile, a wave absorber comprising a foamed polyurethane resin has an advantage in that it is usable in a high-frequency zone because the foamed polyurethane resin has small cell diameters. However, this wave absorber has the following problems. That is, a foamed polyurethane resin has cells of nonuniform diameter and a large number of cell walls in skeleton of foamed resin, and this makes impossible the uniform dispersion of dielectric loss material (e.g. carbon black) in the foamed resin, by a process of impregnating the foamed and molded polyurethane resin with a latex containing a dielectric loss material and then drying the impregnated material, which process is necessary to produce a wave absorber comprising a foamed polyurethane resin.
In any wave absorber, an electric wave entering the absorber is converted into heat energy by dielectric loss, whereby the electric wave is absorbed by the absorber. In a wave absorber comprising a foamed polyethylene resin, since the polyethylene resin has a low softening point, heat build-up in the resin invites deformation or collapse of the foamed resin; moreover, since the polyethylene resin is flammable to inflammable, heat build-up in the resin may cause fuming, fire-catching, combustion, or harmful gas generation. These problems have become serious in recent years because the output of electric waves is becoming larger.
In order to alleviate the above-mentioned problems, there were proposed a method of producing a wave absorber by packing inorganic particles tightly (Japanese Patent Application Laid-Open Kokai! No. 5-243781) and a method of producing a wave absorber by using a foamed phenolic resin (Japanese Patent Application Laid-Open Kokai! No. 6-314894). However, these methods have problems as well. In the former method, the cell diameters are large as in the above-mentioned prefoamed polystyrene resin beads and the wave absorber produced is applicable only in a low-frequency zone. In the latter method, it is necessary to use a flame retardant of phosphorus or halogen type, giving a heavy foamed material (density: about 100 kg/m.sup.3); this, together with the pyramid shape of the wave absorber produced, gives an oppressive feeling to the users of the wave absorber.